string changing v2

#include<stdio.h>
int main()
{
    int i,len,val,f=1;
    char a,b,s[100];
    scanf("%[^\n]s",s);
    scanf(" %c %c",&a,&b);
    len=strlen(s);
    for(i=0;i<len;i++){
        if(s[i]==a){
            s[i]=b;
            break;
        }
    }

    for(i=0;i<len;i++){
        if(s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]== '/' || s[i]=='^'){
            if(isdigit(s[i-1])&&isdigit(s[i+1])){
                f=0;
                break;
            }
        }
    }
    if(f==1){
        printf("%s\n",s);
        return 0;
    }
    for(i=0;i<len;i++){
        if(s[i]=='^'){
            val=pow(s[i-1]-'0',s[i+1]-'0');
            break;
        }
        if(s[i]=='+'){
            val=(s[i-1]-'0')+(s[i+1]-'0');
            break;
        }
        if(s[i]=='-'){
            val=(s[i-1]-'0')-(s[i+1]-'0');
            break;
        }
        if(s[i]=='*'){
            val=(s[i-1]-'0')*(s[i+1]-'0');
            break;
        }
        if(s[i]=='/'){
            val=(s[i-1]-'0')/(s[i+1]-'0');
            break;
        }
    }
    printf("%d\n",val);
    return 0;
}