//暴力
#include <iostream>
#include <cstring>

using namespace std;

const int N = 110,M = 50;

int f[N][M][M][M];//f[i][j][k][d]=min(f[i-1][j-b[i]][k][d],f[i-1][j][k-b[i]][j][d])
int a[N];
int b[N];
int sum=0;

int main()
{
    memset(f,0x3f,sizeof f);
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        cin>>a[i]>>b[i];
        sum+=b[i];
    }
    if(sum%3!=0) cout<<"-1"<<endl;
    else
    {
        f[0][0][0][0]=0;
        for(int i=1;i<=n;i++)
            for(int j=0;j<=sum/3;j++)
                for(int k=0;k<=sum/3;k++)
                    for(int l=0;l<=sum/2;l++)
                    {
                        if(j>=b[i]) f[i][j][k][l]=f[i-1][j-b[i]][k][l]+1-(a[i]==1);
                        if(k>=b[i]) f[i][j][k][l]=min(f[i][j][k][l],f[i-1][j][k-b[i]][l]+1-(a[i]==2));
                        if(l>=b[i]) f[i][j][k][l]=min(f[i][j][k][l],f[i-1][j][k][l-b[i]]+1-(a[i]==3));
                    }
        cout<<f[n][sum/3][sum/3][sum/3]<<endl;
    }
    return 0;
}
//错误代码
//不能去掉一维，因为当前本身就去掉一维，f[i][j]一定被当前维度更新
//后面更新的就错误了
#include <iostream>
#include <cstring>

using namespace std;

const int N = 110,M = 510;

int f[M][M];//f[i][j][k][d]=min(f[i-1][j-b[i]][k][d],f[i-1][j][k-b[i]][j][d])
int a[N];
int b[N];
int s[N];
int sum=0;

int main()
{
    memset(f,0x3f,sizeof f);
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        cin>>a[i]>>b[i];
        sum+=b[i];
        s[i]=s[i-1]+b[i];
    }
    if(sum%3!=0) cout<<"-1"<<endl;
    else
    {
        f[0][0]=0;
        for(int i=1;i<=n;i++)
            for(int j=sum/3;j>=0;j--)
                for(int k=sum/3;k>=0;k--)
                    {
                        if(s[i]-j-k>=b[i]) f[j][k]=min(f[j][k],f[j][k]+1-(a[i]==3));
                        if(j>=b[i]) f[j][k]=min(f[j][k],f[j-b[i]][k]+1-(a[i]==1));
                        if(k>=b[i]) f[j][k]=min(f[j][k],f[j][k-b[i]]+1-(a[i]==2));
                    }
        if(f[sum/3][sum/3]==0x3f3f3f3f)cout<<"-1";
        else cout<<f[sum/3][sum/3]<<endl;
    }
    return 0;
}

//当j,k固定时，l则固定，因此可以用前缀和优化掉一维
//注意不一定为0x3f3f3f3f可能大于0x3f3f3f3f
#include <iostream>
#include <cstring>

using namespace std;

const int N = 110,M = 510;

int f[N][M][M];//f[i][j][k][d]=min(f[i-1][j-b[i]][k][d],f[i-1][j][k-b[i]][j][d])
int a[N];
int b[N];
int s[N];
int sum=0;

int main()
{
    memset(f,0x3f,sizeof f);
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        cin>>a[i]>>b[i];
        sum+=b[i];
        s[i]=s[i-1]+b[i];
    }
    if(sum%3!=0) cout<<"-1"<<endl;
    else
    {
        f[0][0][0]=0;
        for(int i=1;i<=n;i++)
            for(int j=0;j<=sum/3;j++)
                for(int k=0;k<=sum/3;k++)
                    {
                        if(j>=b[i]) f[i][j][k]=f[i-1][j-b[i]][k]+1-(a[i]==1);
                        if(k>=b[i]) f[i][j][k]=min(f[i][j][k],f[i-1][j][k-b[i]]+1-(a[i]==2));
                        if(s[i]-j-k>=b[i]) f[i][j][k]=min(f[i][j][k],f[i-1][j][k]+1-(a[i]==3));
                    }
        if(f[n][sum/3][sum/3]>0x3f3f3f3f)cout<<"-1";
        else cout<<f[n][sum/3][sum/3]<<endl;
    }
    return 0;
}